This invention relates to a multilayer type orientation-modified highly stretched film to be provided for use primarily in wrapping material. More particularly, it pertains to a multilayer film excellent in heat resistance, sealability as well as stretchability, comprising a layer structure of at least three layers, with at least one layer containing a specific mixed composition (M layer), a layer comprising at least one polymer selected from crystalline polypropylene and crystalline polybutene-1 (H layer), and a surface layer (S layer), wherein the whole layers are stretched in at least one direction to be highly oriented, followed subsequently by application of a specific heat treatment to modify its orientation.
The film of the invention is useful for various wrapping purposes such as stretch wrapping, stretch-shrink wrapping, shrink wrapping, etc. or otherwise for skin packing, cohesive wrapping or other wrapping or packaging uses. While the invention is not particularly limited in its use, it is particularly suitable for stretch wrapping and stretch-shrink wrapping, and the following description is made primarily by referring to these uses as an embodiment of practical applications of the present invention.
Packages formed with films are manufactured by a good number of methods which utilize to advantage the characteristics of the films such as the bag sealing method, twist wrapping method, thermal shrink wrapping method, cohesive wrapping method by use of specific films represented by Saran Wrap (product by Asahi Kasei Kogyo K.K.), stretch wrapping method, skin packing method, etc. These methods require respective wrapping characteristics. For each packaging method, therefore, it is generally practiced under the present situation to select a film whose basic material, composition, form and characteristic attributes best suit the wrapping characteristics of the particular method employed.
The shrink wrapping method, on the principle of full use of the heat shrink property of a film which has been stretched to acquire a specifically set orientation, comprises the steps of loosely pre-wrapping and sealing a given article subjected to wrapping so as to enclose the article with the film and, thereafter exposing the film to a heat medium such as a current of hot air, an infrared ray, or hot water and the like, thereby causing the film to shrink and come into tight contact with the overall contour of the article. This method is characterized by the fact that the produced package has a beautiful appearance which adds to the commodity value of the wrapped article. In addition, the package keeps the contents hygienic, permits shoppers to touch and usually examine the quality of the contents, keeps the contents tightly in position regardless of shape or whether there are a plurality of pieces in one package, and provides the content with ample protection against vibrations and impacts. Compared with the stretched wrapping method which is used extensively in supermarkets, the shrink wrapping method provides high speed wrapping of the article. Also, this method permits the wrapping of an article with a complicated shape which cannot be wrapped by the stretch wrapping method and also, the wrapping of an article without a vessel such as a tray. While there is another advantage enabling tighter wrapping, there is involved the disadvantage that the film must be heated sufficiently until the film is shrunk. On the other hand, the shrink films generally employed in the art are not suited for the purpose of stretch wrapping because of drawbacks such as small elongation of the film itself, breaking of the film when overly stretched, too strong a stress relative elongation to effect elongation easily and substantial absence of self-stickiness. These are the drawbacks and problems of the shrink films of the prior art.
On the other hand, as is well known, stretch wrapping has been popularly employed in recent years in super markets for wrapping vegetables, fruits, fresh feedstuffs, meat and various cooked foods. The advantages are that is it high in fitness capable of responding to uneven shapes or sizes of the article to be wrapped without causing the film to have residual permanent deformation or creases when stretching the readily elongated film. Also, the film can be easily fixed by light pressure or heat setting of the drawn film without sagging of the packages. In addition, it has an appropriate air permeability for fresh foods thereby preserving freshness and preventing a reduction in weight of the article to be wrapped. Furthermore it can keep the article to be wrapped hygienic and permits shoppers to confirm the quality by vision and touch. The wrapped article is also beautiful in appearance thereby adding greatly to the commodity value of the article to be wrapped. Both the inexpensive manual wrapping machine and the highly efficient automatic machine are easily available and no heat is given to the article to be wrapped.
In the prior art, however because of employing an unstretched film having a high elongation of the film, there are involved numerous problems. The mechanical strength of the film is low resulting in frequent breaking of the film or formation of holes when sealing the folded films at the bottom of a tray. The operability (mechanical suitability, manual workability) is poor due to a low modulus of the film, whereby the film cannot be made extremely thin in view of its quality, and thus thick films must be used.
Heretofore, with an aim to develop a film having properties equal to or better than those of the plasiticized polyvinyl chloride film (containing 30 to 33 wt. % of plasticizer), various manufactures have developed films comprising various resins (e.g. EVA; ethylene-vinyl acetate copolymer, 1,2-polybutadiene resin, L-LDPE:linear LDPE, etc.), which are substantially unoriented films consisting of other kinds of resins, to challenge the market. However, under the present situation, none of these attempts are considered to be successful. Further, while development of a complete film other than plasticized polyvinyl chloride film is rapidly desirable because of the problems in pollution and hygiene at the present time, only an insufficient film which is poor in handling is now being used as a trial. Such a film cannot be handled with ease and is therefore not welcomed at the working site of wrapping. The amount of such a film employed is very small and far from providing a substitute for the polyvinyl chloride film. These films are those of high quality to be utilized primarily for foods, as distinguished from those films having low levels of the requisite characteristics to be generally utilized for industrial applications, such as pallet stretch film.
The manufacture of a drawn film of polypropylene on the other hand, is accomplished by a method comprising the steps of melt extruding the polymer resin through an extruder die, quenching the extruded tubular sheet, reheating the cooled tubular sheet at a high temperature within the range of e.g. from 150.degree. to 160.degree. C. , and forcing air into the inner cavity of the tubular sheet. In the case of a drawn film of low-density polyethylene, a similarly extruded tubular sheet of the polymer resin is biaxially drawn in an effort to set a high degree of molecular orientation in the film. In the course of the drawing, however, the sheet bursts, making a manufacture of film hardly practicable from the technical point of view.
Because of the above mentioned difficulty, the generally method is a direct inflation method which comprises the steps of extruding the polymer resin at a temperature within the range of from 180.degree. to 220.degree. C., for example, and subsequently causing the extruded sheet, by means of a proper form of air, to be simultaneously cooled and inflated to a prescribed size.
While this method is characterized by producing a film easily at low cost, it is impossible to set satisfactory molecular orientation by stretching, because flowing will readily occur between molecules. Also, the optical properties are very inferior. Accordingly, for use in shrink wrapping, the resultant film has low heat shrink percentage, low heat shrink stress and shrinks at rather high temperatures. Therefore, such a film can be used only in special uses with increased film thickness. For the purpose of improving the above drawbacks, it has been proposed to prepare a film of low density polyethylene according to the steps of extruding a film of low density polyethylene, irradiating the resultant film with a high energy radiation to effect partial crosslinking thereof, stretching the film by heating to a high temperature (e.g. 140.degree. C.) in excess of the melting point, whereby flowing between molecules can be prevented to sufficiently set molecular orientation. However, the extent of low temperature shrinkability, which is one of the most important characteristics in the shrink packaging method, is low and the film obtained readily bursts with small elongation. The film produced according to this process is small in elongation and too high in stress on elongation to 100 or 200% (frequently not elongated to such percentages). Therefore, it is a film which cannot entirely be utilized for stretch wrapping film, and is only utilized in a part of shrink wrapping as a film shrinkable at high temperature.
As a new category of wrapping films, a variety of composite multilayer films have been known.
Recently, to meet the demand of higher requisite characteristics, composite (multilayer) films have been increasingly developed. For example, there exist generally a large number of composite films having other resins laminated by melting on unoriented films or oriented films.
For example, various kinds of films and combinations are chosen depending on uses, such as a film improved in heat sealing property by fusion lamination of other resins on an unoriented polypropylene according to the casting method (called as C.PP) or an oriented propylene (O.PP) or a film improved in barrier performance prepared by coating with a polyvinylidene chloride type latex (called as K coat film), etc.
On the other hand, it has also generally been known in the art to produce a co-extruded film of unoriented type by melting various kinds of resins separately in respective extruders, permitting the molten resins to be confluent within a multilayer die to be combined and extruded therethrough, followed by cooling, into films and sheets.
However, in the case of a stretched multilayer film, in the first place, each layer constituting the multilayer product differs in optimum extruding conditions, stretching conditions, etc. depending on the resin employed, even if it is desired to obtain a film stretched to a high degree. According to the technique of the prior art, unfavorable phenomena occur such as uneven thickness, vertical streak, puncture, bursting, peel-off of respective layers and whitening due to interface roughening occurred, whereby no satisfactory product can be obtained. Also, even when a small piece of the film may be obtained, such a product is far from the film having intended characteristics under the present situation. It has been deemed to be very difficult to solve these drawbackes. Moreover, it has been deemed to be entirely impossible to use an oriented film as a stretchable film which requires stretchability and no such thought has ever been conceived. The present inventors have invented U.S. Pat. Nos. 4,277,578, 4,399,181 and 4,430,378 to overcome the drawbacks of the shrink films as described above. The films proposed by these patents are more suited for uses of shrink wrapping. Further, as a film more suited for uses of stretch-shrink wrapping, the present inventors have invented a film as disclosed in Japanese laid-open Patent Publication No. 175,635/1983. But that film was found to be yet unsatsifactory as a complete stretch film, with respect to its stress balance relative to elongation and also elongation as a whole. Thus, in some cases, it has been frequently utilized primarily with respect to its low temperature shrinkable properties acquired from the cold stretching method, secondarily with less stress imposed on elongation. In other words, the film was utilized as a film having more excellent elongation and low temperature shrinkability as compared with the films of other methods. The present invention has achieved a still more excellent invention which satisfies sufficiently the properties deficient in these inventions and also enlarged its uses. This will be clarified in more detail by comparison with Comparative examples as hereinafter described.
As a more detailed description of the stretch wrapping method, the films to be used commercially for these methods of the prior art are only films made of materials containing a large amount of plasticizers, such as soft polyvinyl chloride resins containing as much as 30 wt. % or more of plasticizers (hereinafter abbreviated as PVC). These films cannot be extruded or endowed with a flexible property to be unsuitable for this kind of use (stretch), unless a large amount of a plasticizer is mixed therein. Thus, since a larger amount of a plasticizer, for example, dioctylphthalate, dioctyladipate, etc. is employed, there are involved various problems such that the amount of water vapor permeated through the films is increased to readily cause denaturation of the article to be wrapped. Moreover the plasticizer is liable to migrate to the article to be wrapped and cause contamination thereof. Cutting of the film by hot wire during wrapping working will cause generation of the gas of the plasticizer and corrosive chlorine type gas which are undesirable from viewpoint of hygiene; and toxic gases are generated during incineration of the used films. Finally, the film is inferior in cold resistance and is less pliable, brittle, liable to burst during storage of the wrapped product at low temperatures and the film roll is heavy.
Whereas films constituted only of high density polyethylenes, low density polyethylenes or polypropylene polymers among polyolefins for general purpose have excellent properties with respect to the drawbacks in pollution as described above, they are devoid of important properties necessary for uses as intended by the present invention. Thus, it has been difficult to provide a practical film for stretch wrapping capable of satisfying the various characteristics as mentioned below with the use of these films. More specifically, the film to be used for stretch wrapping must, at the same time, satisfy all the characteristics as mentioned below. Thus the films must have:
(a) adequate stress relative to an adequate elongation, and also a high elongation at break;
(b) excellent film-to-film adhesion;
(c) adequate delayed recovery characteristic, high deformation recovery, an adequate elastic elongation, and strong in mechanical strength;
(d) adequate slipping characteristic at the surface of a film;
(e) excellent optical characteristics such as transparency and gloss;
(f) adequate gas permeability;
(g) excellent anti-fogging property without retention of water droplets on the surface;
(h) excellent wrapping operability; and
(i) heat resistance during sealing.
For example, when an unoriented film of polypropylene is first drawn for stretch wrapping, only a certain portion is elongated, whereby the phenomenon called necking occurs with generation of extreme thickness irregularity. Even after removal of the load, the portion remains in the state as drawn, whereby the wrapped product is greatly damaged in appearance and the intended purpose of wrapping is not achieved. Also, the stretched film is hard, strong and small in elongation, requiring a very great force for elongation up to the point at which the article to be wrapped is broken. Such a film has no tackiness and, even if a plasticizer such as a liquid polybutene or a polybutene with low polymerization degree may be mixed therein, no tackiness can be imparted thereto, unless it is used in an amount of 5 wt. % or higher. If employed in such an amount, since polyolefins lack the ability of retaining plasticizers as possessed by polyvinyl chloride resins, most of the plasticizers will bleed out on the surface to make the film sticky and practically unuseful.
High density polyethylene films are also similarly hard and cannot easily be stretched to give similar results. In fact, they are opaque without luster, thus having no possibility of application.
Low density polyethylene films are relatively softer as compared with those as mentioned above, but non-stretched films suffer also from necking, with the deformation recovery being small, having lower strength, no good transparency and no tackiness. Thus, they are not useful for the purpose of the invention. On the other hand, low density polyethylene films crosslinked with, for example, an electron beam so as to be made readily stretchable, and stretched according to a conventional method, have the same drawbacks as polypropylene, and therefore they also are not provided with properties suitable as the base material for stretch wrapping, as intended by the prevent invention.
In the case of an elastic elastomer having substantially complete deformation recovery comprising a base material such as styrene-butadiene latex or other rubbers, although there is no such phenomenon as necking, there is a problem with respect to optical characteristics and hygiene of foods. In addition, the strength when elongated is approximately in direct proportion to the elongation and the response of relaxation of deformation recovery is effected momentarily. That is the film will be restored to its original state momentarily when it is released in setting the film end portion at the article to be wrapped or at the bottom portion of a tray, whereby the wrapping will be loosened. Thus, this kind of material does not have the desired properties suitable for one of uses intended by the present invention.
Of these polyolefin type films, there are also commercially sold trial samples comprising mainly crystalline 1,2-polybutadiene type or ethylene-vinyl acetate copolymer (EVA), admixed with an anti-fogging agent, tackfier, etc., formed into films in a conventional manner (T-die method, air-cooling inflation method, etc.). However, these films have a number of drawbacks and have not yet commercially been sold in full-scale without reaching the level as substitute for films of the prior art.
The above films cannot eliminate at the same time all of the properties antagonistic to each other such as easy elongation during packaging, heat resistance at the sealed portion, easy sealing, and, further, prevention of bursting during packaging due to deficiency in film strength. Thus, the films consequently have only average and incomplete properties. For example, in the case of an EVA type film, when the content of vinyl acetate (VAc) in EVA is increased, the film turned around the bottom portion of a vessel will tend to be molten by heat and burst readily. For this reason, in order to prevent such a tendency, the thickness of the film must be increased from 16.mu. to 20.mu., or 22.mu. or 24.mu.. As a consequence, drawbacks occur such that the film becomes hardly elongatable, the films become less sticky before sealing due to the rubbery elastic components and the increased film thickness makes sealing more difficult. Moreover, there is also involved the problem of disadvantageous increase of cost. Then, as a next measure, there is adopted the method of incorporating, for example, a low density polyethylene (particularly of the linear type), a polypropylene or a rubber into EVA. By such a method, however, no great extent of improvement can be brought about. On the contrary there is the tendency that important characteristics such as transparency, gloss, etc. may be lowered to further pose additional problems.